1. Field of the Invention
The present invention relates to an apparatus for voltage multiplication wherein the output voltage has a relatively low dependence on the supply voltage, such that comparatively simple and good regulation of the output voltage can be produced even in integrated circuits.
2. Description of the Prior Art
In order to program non-volatile memories, such as flash EEPROMs, a "high voltage" up to approximately 30 V generated by so-called voltage pumps is typically used. These voltage pumps operate on the principle of capacitive voltage multiplication and have one MOS diode and one capacitor per pump stage. Regulated voltage pumps which ensure a large constant internal rated voltage are employed in integrated circuits in which major fluctuations in the supply voltage may be allowed. These charge or voltage pumps must be designed in such a way that, even with small supply voltages such as 2.5 volts the internally required rated voltage, of 5 volts for example, is still reached. The result of this, however, is that at high supply voltages of approximately 5 or 6 volts, comparatively very high voltages of 20 or 30 volts are reached in a relatively short time, which leads to considerable regulation problems.
The publication by A. Umezawa et al., "A 5-V Only Operation 0.6 .mu.m Flash EEPROM with Row Decoder Scheme in Triple-Well Structure", IEEE Journal of Solid State Circuits, Vol. 27, No. 11 (1992), discloses an apparatus for voltage multiplication with high-voltage PMOS transistors and additional boost transistors. However, this apparatus is not suitable for integrated circuits in which severe fluctuations in the supply voltage may be allowed.
The European Application Document 0 350 462 discloses regulation of the output voltage of a voltage multiplier in which the clock signals originate from a ring oscillator whose frequency depends on the output voltage.
The European Application Document 0 135 889 discloses a circuit for voltage multiplication in which the substrates of the p-channel transistors are permanently connected to the supply voltage and the substrates of the n-channel transistors are permanently connected to the reference-earth potential.
A object of the present invention, therefor, is to specify an apparatus for voltage multiplication which has a low dependence of the output voltage on the pump voltage or supply voltage and which is suitable for the widest possible supply voltage range.